This invention relates to the art of paper coating, and more particularly to method and apparatus by which the size and quantity of entrained bubbles are controlled in the coating liquid with the lowest practical pressure drop and with a low reject rate.
Such coatings which can be troubled by gas entrainment include mineral pigmented printing coating, general purpose clay coatings, coating for thermal and facsimile machines, and other specialty aqueous and solvent-based coatings.
Gas entrainment in coating liquids has several deleterious effects on the practice of coating. It prevents the practice of the jet coating method by causing coating skips or requires an excessively high recirculation rate to overcome the problem. It creates small coating voids in the final product. It can cause the coating to be discarded when the gas concentration is too high for good coating flow. In addition to requiring a high recirculation rate, the current air removal devices induce a high pressure drop and require a high reject rate, further increasing the capital and operating costs of the coating system.
Large bubbles can bridge the orifice slot of the coater, and cause a skip in the coating. The coating station operator is forced to increase the width of the die lips or the orifice slot so that it will pass the largest expected entrained bubble without creating a void in the shot flow. However, this results in the application of a heavier coating, and the requires the doctoring off of a larger percentage of the coating. This coating is returned for reuse, and the handling increasing the coating gas content. The excess coating also loads the doctor blade system. Inevitably, more coating material is used, and later discarded when it is no longer practical to separate the gas. Smaller bubbles which pass the die lips can also pass under the doctor blade, and cause a void defect in the paper.
Gas of any kind, but particularly in the form of a large quantity of very small gas bubbles, also is detrimental to the quality of the coating. While small bubbles, also known as microbubbles, can pass the die and blade without localized adverse effects, they unbalance the rheology and change the viscosity of the coating. The coating weight and quality suffer. As noted above, when the coating becomes laden, the coating material may become useless.
The paper converting and coating industry has addressed these problems in a number of manners, including the design and use of cyclone or vortex type centrifugal separator systems, with some success. However, in such systems, certain problems persist. These problems include high pressure drops or a sudden onset of failure at certain upper limit flow rates or high reject rates. Such systems require a detailed knowledge of the anticipated flow rates. These excess rejection rate in the separator, excess pressure drop, and an inability to extract fine bubbles consistently or at all, have been typical. Low efficiency of bubble separation translates into low production rates and/or high costs for excess pumping capacity and wasted coating material.